Improve error message for if with an else part while in switch

Improve error message for if with an else part while in switch

While writing some match.pd code, I was trying to figure
out why I was getting an `expected ), got (` error message
while writing an if statement with an else clause. For switch
statements, the if statements cannot have an else clause so
it would be better to have a decent error message saying that
explictly.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* genmatch.cc (parser::parse_result): For an else clause
	of an if statement inside a switch, error out explictly.